1. Field of the Invention
This invention relates to a circuit for providing rapid detection of a malfunction in the form of a missed commutation in an inverter circuit of the type used in induction cooking or heating systems in order to temporarily interrupt the operation of the inverter until the cause of the malfunction is removed to prevent damage to critical circuit components.
2. Description of the Prior Art
One well known induction cooking system is disclosed in U.S. Pat. No. 3,814,888, and utilizes inverter circuits to convert DC power supply voltage to an ultrasonic output frequency wave for driving an induction heating coil. The inverter circuit utilizes a pair of controlled solid state power devices which are rendered conductive alternately for intervals of conduction at a rate to produce the ultrasonic output frequency wave. In such an arrangement if a commutation failure occurs in one of the solid state power devices the result is a short circuit across the DC bus at the inverter input. Some means must be provided to open this short or to isolate the inverter in order to prevent destruction of the power control devices and other circuit components. A similar inverter circuit for use as an induction cooking control is discussed in RCA Application Note AN-6456.
One common method used to protect against missed commutations in an inverter is a fast-acting fuse. However, since the blown fuse requires a maintenance action to restore operation of the inverter, it can only be employed in connection with a circuit design in which commutation failures are very infrequent. Such systems require that a large margin be provided in the energy storage components to assure that commutation will reliably occur even during transients in source voltage or load impedance. These systems are consequently excessively expensive.
Other prior art systems for preventing commutation failure employ sophisticated detection schemes which require dual isolated power supplies, operational amplifiers and a comparator. Such systems are impractical for use in high volume applications such as induction cooking ranges where load impedance is very erratic, reliability without maintenance is essential and economic factors prevent the use of large margin components.
In U.S. Pat. No. 4,068,294, there is disclosed a circuit for detecting a missed commutation in an inverter by, in effect, measuring the time lapse between the occurrence of a turn-on triggering pulse to a turned-off SCR and a sensed turn-off of the non-triggered SCR. This arrangement uses a photodiode and phototransistor which operates as an optoisolator, and contemplates a period when two SCR's which are connected across the DC bus in the same leg of the inverter are concurrently on and is different in this respect from the invention disclosed herein. The timing measurement of this patent is accomplished by an ANDing circuit which provides a disconnect signal if the series-connected SCR's in the same leg of the inverter are both ON a preselected time after the occurrence of the turn-on trigger pulse to one of the SCR's. Because it requires a very expensive center tapped commutating interval current limiting reactor and additional components, it is not economically feasible for high volume production.
In U.S. Pat. No. 3,821,630, there is disclosed a commutation failure detection scheme comprising only a single power switching device. This arrangement uses a current switch in series with the switching device which is switched concurrently with the SCR. A comparator is used to continuously monitor the instantaneous voltage across the SCR. Trigger pulses to the current control device are inhibited when the voltage across the SCR is above a predetermined reference level.